1. Field of the Invention
The present invention relates to a specimen holder for use with an electron microscope (EM) and, more particularly, to an EM specimen holder capable of obtaining a specimen image even at high tilt angles. In recent years, tomography, that is, a three-dimensional image reconstruction technique, has attracted attention chiefly in biological fields. In tomography, images of a tilted specimen are processed. If the amount of information about the specimen is increased, a more accurate three-dimensional image can be built. The present invention relates to an EM specimen holder used for acquisition of the information.
2. Description of Related Art
FIG. 5 shows the outer appearance of a prior art specimen holder for use with an electron microscope. The entire holder is shown. The body of the specimen holder is indicated by numeral 1. The holder has a handle 2. A retainer 3 (specimen-holding portion) for holding a specimen 4 is mounted at the front end of the body 1 of the holder. The specimen 4 is held to the retainer 3. The body 1 of the specimen holder has a tilted axis 5. The body 1 of the specimen holder is mounted to a goniometer (not shown) for an electron microscope. The body 1 of the holder is tilted about the tilted axis 5 by the tilting mechanism of the goniometer. Directions of tilt are indicated by the arrows A. As shown, the specimen holder is tilted in a clockwise direction and in a counterclockwise direction.
FIG. 6 is an exploded perspective view of the prior art retainer. In both FIGS. 5 and 6, like components are indicated by like reference numerals. In FIG. 6, a plate member 10 is used to attach the specimen 4 having a diameter of about 3.0 to 3.2 mm and a thickness of about 50 μm. A hole 10a is formed around the front end of the plate member 10. The hole 10a provides a reference in searching the whole specimen for a desired field of view. A circular groove 10b is formed around the front end of the plate member 10 and used to determine the position of the specimen. A notch 10c is formed at the foremost end of the plate member 10 and offers an insertion port into which the front ends of tweezers are inserted when the specimen is taken out.
A leaf spring 11 is used to hold down the specimen 4. The leaf spring 11 is mounted into a threaded hole 10d formed in the plate member 10 by a screw 12. A method of holding the specimen with the retainer constructed as described so far is next described.
First, the specimen 4 is placed along the circular groove 10b formed in the plate member 10. Then, the leaf spring 11 is held to the plate member 10 with the screw 12. As a result, the specimen 4 is held to the plate member 10 by the force of the leaf spring 11.
A known device of this kind uses a cartridge whose body has a front-end portion in which a meshed portion is formed. A slice of a specimen is picked up by this meshed portion (see, for example, JP-A-8-17381 (paragraph 0010; FIGS. 1 and 2)). Furthermore, a holder for carrying a slice of a specimen while maintaining it at a low temperature is known (see, for example, Japanese Patent No. 3,605,119 (from page 4, line 32 to page 5, line 4; FIG. 1)). The slice of the specimen includes a specimen slice grid made of a thermally conductive material. The specimen slice grid has plural grid bars intersecting each other so as to form grid openings. Each of the grid openings has a length and a width. The longitudinal direction of each grid opening is substantially perpendicular to the longitudinal axis of the holder body. The length of each grid opening is greater than the width of each grid opening. The slice of the specimen can be rotated through an angle of at least 90° in both directions from the horizontal direction.
FIGS. 7A-7D illustrate the effective field of view when the prior art retainer is tilted. In all of FIGS. 6 and 7A-7D, like components are indicated by like reference numerals. The direction in which the retainer is observed is shown in FIG. 7B. FIG. 7A shows the state in which the tilt angle is 0°. The specimen 4 is placed on the plate member 10. The specimen 4 is held from above with the leaf spring 11. When an electron beam EB is directed at the specimen from above it, an image B is focused onto a fluorescent screen. The diameter of the effective field of view is 2 mm.
When the tilt angle is 60°, the specimen 4 is tilted at an angle of 60° as shown in FIG. 7C. As a result, an elliptical field of view can be secured as indicated by C on the image focused on the fluorescent screen. FIG. 7D shows the state in which the tilt angle is 80°. In this case, the field of vision is blocked by the leaf spring 11 and by the wall defining the circular groove 10b. The image on the fluorescent screen is invisible. When the specimen is taken out, it is difficult to bring tweezers to capture the specimen. There is the problem that the specimen is damaged or scratched. In addition, it is necessary that the specimen can be placed in position easily and reliably when the specimen is mounted. Moreover, it is necessary that the specimen can be taken out easily and reliably.